| Lignite is a kind of low-rank coal with high moisture content,complex pore structures and rich oxygen-containing functional groups for the short time of forming coal and the low degree of coalification.Besides,the trend of reabsorption and spontaneous combustion after dehydration and the low slurryability seriously restrict the large-scale industrial application of lignite.So the lignite should be upgraded with an economic,efficient and rapid method to increase its interfacial stability.Treatment under microwave irradiation,heating the water molecules directly and making them be vaporized out,is an effective upgrading technique.Compared to conditional heating dehydration techniques,the transferring directions of water and heat are the same in the microwave irradiation process and microwave is a “cold heat source”,which protected from the oxidation of lignite in a low-temperature environment.In addition,the specific pattern of heat and mass transfer contributes to the eliminating of pore structures and the removal of oxygen-containing functional groups,which enhances the interfacial stability of lignite.This thesis focuses on the migration characteristics of moisture and the changes of physicochemical structures as well as the re-absorption characteristics,the spontaneous combustion characteristics and the slurryability of lignite treated under microwave irradiation.The research is as follows:1)Ximeng lignite is a kind of low-rank coal with high moisture content,high volatility content,low fixed carbon content,low calorific value and high oxygen content according to the proximate and ultimate analysis.Internal water shows stronger combination with lignite than external water based on the analysis of low-field nuclear magnetic resonance(LFNMR).The migration rates of water increase under higher power levels.In addition,the dehydration rates drop down with less moisture content for the water acts as absorbers and the residual water shows stronger combination with lignite.Carbon-absorbers can enhance the removal of water in lignite.Especially speaking,petroleum coke is an ideal carbon-absorbers for it can make the carbon-based slurry with lignite.2)The experiment to modify microwave nominal power levels with water load method shows the differences between the actual power levels adsorbed by water load and nominal power levels,which resulted from the air holes in the furnacechamber,the hole at the bottom,the heat exchange in the process of heating water and so on.The actual energy consumption of different forms of water in lignite can be calculated based on the actual power levels.The value of removal energy consumption of external water on the surface of lignite is similar to that of free water in a common situation for this part of water combines with lignite weakly.The value of removal energy consumption of external water in macropores increases for this part of water attaches to lignite a little more intensive and the absorber of microwave energy decreases as water migration.Removal of internal water consumes lots of energy for the strong force between water molecules and lignite such as capillary force.3)Complex pore structures,high capacity of pore volume and large specific surface area of lignite are analyzed with nitrogen adsorption instrument.All in all,the values of pore volume and specific surface area go down in the microwave irradiation process.Shrinkage stress from the migration of large amounts of water leads to the collapsing and shrinkage of pores.Besides,the plugging effect of pieces of collapsing and melted gel materials on the pore structures contributes to the down trend.Initially,migration free water in the macropores in of lignite water leads to the collapsing of the macropores.As a result,the values of the pore volume and the specific surface area drop significantly.The process from shrinkage to collapsing causes that the decrease of the value of the specific surface area is relatively lagging behind the decrease in the value of the pore volume.After that,the value of specific area goes up reversely as some macropores becoming into mesopores.Then the values of the pore volume and the specific surface area decrease again for the collapsing and shrinkage of mesopores.At last,the removal of molecular layer bound water contributes less to the modification of pore structures.4)The variation characteristics of the oxygen-containing functional groups are researched by the method of acid-base titration The results show that compared with the traditional thermal drying method,the microwave energy heat water in the lignite directly and the temperature of the lignite surface is low,which ensures lignite will not be seriously oxidized.In the initial stage of microwave irradiation,the microwave energy absorbed by water transforms into the internal energy and the surface of the lignite forms into a high-temperature environment.The interface of the lignite is partially oxidized and the content of oxygen-containing functionalgroups increase lightly.Since then,the interior of the lignite forms a high temperature and high-pressure environment,which lead to the removal of some oxygen-containing functional groups in the lignite.Especially speaking,the oxygen-containing functional groups bounded with the water to remove further with the migration of the molecular layer bound water.On the whole,the aroma of lignite is deepened and the structure shows graphitization during the microwave irradiation through the XPS.The results of correlation analysis of the oxygen-containing carbon groups are similar to those of the chemical titration.5)By drawing the sorption(desorption)isothermal curves of lignite treated under microwave irradiation,it can be found that the lignite has a wide pore size distribution,including micropores,mesopores and macropores.Among them,the mesopores and the macropores contribute most.The macropores make the sorption isothermal curve show a tendency of no adsorption upper limit and the capillary condensation caused by the mesopores lead to the appearance of the adsorption hysteresis loop.The reabsorption kinetics of lignite treated under microwave irradiation satisfies the double exponential decay model,indicating that the recovery process includes the diffusion stage and the migration stage.The environment decides the diffusion mainly.The diffusion becomes faster and the chance of water molecules colliding and adsorbing at the interface of lignite increases for the augment of the partial pressure of the water vapor when the relative humidity of the environment goes up.Reabsorption behavior of lignite treated under microwave irradiation is the process of water molecules adsorbing at the interface of lignite again,so the content of final equilibrium moisture of the lignite in this process is linearly correlated with the value of the specific surface area.The value of pore volume has a positive correlation with the final equilibrium moisture for pores serve the storage space for water molecules.The lignite,treated under microwave irradiation,shows less trend to reabsorption for its less specific surface area and low pore volume as the pore structures shrink and collapse in this process.Relationships between the content of oxygen-containing functional groups and the content of final equilibrium moisture show a different trend compared with the pore structures when the lignite is treated under higher power levels.This is because the treatment with the higher power levels favors the removal of the oxygen-containing functional groups,but the effect of the decrease of the specific surface area reverses at the later stage of the irradiation process treated under thehigher power levels.Compared to the lower power levels,more mesopores appears and the value of specific surface area rises for the stronger drying stress under higher power levels,which offsets the removal content of oxygen-containing functional groups and leads to the larger amount of reabsorption.6)Compared to the ways of a certain power level and first using low power level and then using high power level,the amount of water reabsorbed by the lignite after microwave irradiation decreases significantly through the way of first using high power level for rapid dehydration and then low power level for upgrading.Interfacial stability of the lignite is also improved by the combination of the pre-dehydration with the thermal drying and the microwave irradiation for upgrading.7)TG/DTG-DSC curves show that the lignite treated under microwave irradiation has experienced the stage of weight loss for dehydration,the stage of weight growth for oxygen absorption and the stage of oxidation-decomposition before the spontaneous combustion like the raw lignite.The stage of weight loss for dehydration is an endothermic process,showing a slight downward and wide endothermic peak.During the stage of weight growth for oxygen absorption,the combination of some active groups at the interface of the lignite and the oxygen molecules releases a small amount of heat and leads to the DSC curves going up.During the stage of oxidation-decomposition,the reaction releases a lot of heat and makes DSC curves go up at a high speed.After that,the heat flow rate increases while its increasing rate slows down during the reaction progresses.8)Calculating the kinetic equation of oxidation-decomposition stage of lignite by using the model-free method and the numerical analysis based on the composite Simpson's rule and linearization are more reliable in theory and higher in calculation precision.The oxidation-decomposition stage of lignite is divided into two sub-stages.The first sub-stage satisfies the one-dimensional diffusion model.The reaction is controlled by the diffusion rate of the pyrolysis substance and oxygen in the pipe channel formed in the pore structures.The second sub-stage satisfies the contraction of the cylinder(area)in the phase boundary reaction and occurs mainly on the surface of the lignite pore structures,which is dominated by the reaction.The chemical structures of lignite will be different according to its interfacial properties in the process of low-temperature oxidation,that is,the spontaneous combustion characteristics and the interfacial structures of lignite areclosely related.Fewer macropore content is not conducive to the diffusion of substances of pyrolysis and oxygen in the lignite.Lignite irradiated under 600 W doesn't react intensively and shows a higher burning point for its low content of macropore prohibit the diffusion of decomposed components and oxygen and small surface area leads to less reaction area.That is to say,the spontaneous combustion tendency is weaker and the interface are more stable after treated under 600 W.9)Microwave irradiation treatment can remove some acidic oxygen-containing functional groups on the interface of lignite and weaken the water holding capacity of lignite.As a result,the hydration film structure of lignite surface in the coal water slurry is thinner and the free water content in the slurry increases.The solid loading of slurry increased by 3.7% after 12 min irradiation.In addition,the change of the physical and chemical structure of the lignite particles after microwave irradiation makes the rheology and stability of the slurry be optimized as a whole.However,the slurry's stability making with lignite irradiated after a longer time(12 min)gradually worsens during the test process.The particle size distribution model based on fractal theory can improve the packing efficiency of the particle system in coal water slurry.The two-grade fractal model can effectively guide the gradation of lignite water slurry;thus the maximum solid loading is increased by 2.9%.Compared with the method of changing the particle size distribution and optimizing the packing state,the solid loading of coal water slurry is increased by 3.7% for the modification of interfacial physiochemical structures and the water holding capacity is reduced through microwave irradiation.The effect of this method is obvious and no UFS leads to shorter grinding time and lower cost,but it needs to be carried out under the microwave field,which consumes more power. |
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